1. Field of the Invention
The present invention relates to an optical pickup apparatus that performs an operation of reading a signal recorded in an optical disc or an operation of recording a signal in the optical disc with laser light.
2. Description of the Related Art
An optical disc device has been widespread which can perform a signal reading operation and signal recording operation by applying laser light emitted from an optical pickup apparatus to a signal recording layer of the optical disc.
While the optical disc devices for optical discs called CD or DVD are available in general, there have recently been developed those for optical discs whose recording densities are improved, that is, optical discs of Blu-ray or HD DVD (High Density Digital Versatile Disc) standard.
Infrared light having a wavelength of 780 nm is used, as the laser light for performing the reading operation of a signal recorded in a CD standard optical disc, and red light having a wavelength of 650 nm is used as the laser light for performing the reading operation of a signal recorded in a DVD standard optical disc.
In contrast to such CD-standard and DVD-standard optical discs, laser light having a short wavelength such as blue-violet light having a wavelength of 405 nm is used as the laser light performing the reading operation of a signal recorded in the Blu-ray standard or HD DVD standard optical disc.
The thickness of the protective layer provided on the upper face of the signal recording layer of the Blu-ray standard optical disc is 0.1 mm, and the numerical aperture of the objective lens used for the reading operation of a signal from the signal recording layer is specified at 0.85.
On the other hand, the thickness of the protective layer provided on the upper face of the signal recording layer of the HD DVD standard optical disc is 0.6 mm, and the numerical aperture of the objective lens used for the reading operation of a signal from the signal recording layer is specified at 0.65.
For such an optical pickup apparatus compliant with the optical disc standard with improved recording density, the optical characteristics required in order to improve signal recording quality have become rigorous in response to the improvement of the recording density.
The optical pickup apparatus is configured such that a driving current supplied to a laser diode can be controlled so as to obtain a laser output suitable for reading a signal recorded in the optical disc or recording of a signal into the optical disc.
Also, the optical pickup apparatus is configured such that a focusing control operation can be performed, which is a control operation of controlling a spot of the laser light applied from the optical pickup apparatus so as to be focused on a signal recording layer of the optical disc and a tracking control operation can be performed, which is a control operation of controlling the spot of the laser light so as to follow a signal track.
In the optical disc device, the optical disc is placed on a turntable, which is driven to be rotated by a spindle motor, and is driven to be rotated, however, inclination caused by warping of the optical disc itself, a state of the optical disc placed on the turntable or a mechanical error, may occur in the optical disc. If the optical disc is inclined, inclination of an optical axis of the laser light relative to a signal surface of the optical disc is deviated from an optimal state.
With the increase in the density of signals recorded in the optical disc, the above-mentioned deviation of the optical axis of the laser light relative to the signal surface of the optical disc becomes a significant problem, and thus, there has recently been developed the optical pickup apparatus having a function capable of performing a so-called tilt control operation, that is, an operation of adjusting the inclination of the optical axis of the laser light relative to the signal surface of the optical disc.
In the optical pickup apparatus, an aberration called a spherical aberration or coma aberration occurs, which is caused by a thickness of a protective layer between a laser light incident surface and a signal recording layer of the optical disc and the like, so that operations of reproducing a signal or recording a signal can not be performed normally, and thus, an art for solving such a problem has been developed (See Japanese Patent Laid-Open Publication No. 2006-147069.)
In recent optical discs, there has been introduced a multi-layer optical disc in which a plurality of signal recording layers are provided in order to increase signal recording capacity.
The above-mentioned patent document describes an art relating to an optical pickup apparatus capable of correcting the spherical aberration or coma aberration. However, it is not assumed in the multi-layer optical disc, for example, that a correction operation is performed to a great change in the spherical aberration with an instantaneous change in the thickness of the protective layer when an operation is performed of jumping between the signal recording layers, and thus, there is a problem that an aberration correction operation can not be performed corresponding to the instantaneous change.